The present invention relates to the use as fertilizers of certain ferrocene derivatives, some of which are known, and especially to their use for the prevention and cure of iron deficiency diseases in plants.
It has already been disclosed that ferrocene derivatives can be used for treating iron deficiency anaemias in humans and animals and as antioxidants, anti-knock agents, additives for motor fuels and oils, color pigments, radiation absorbers, insecticides and fungicides (see British patent specification No. 898,633, U.S. Pat. Nos. 3,432,533, 3,535,356, 3,553,241 and 3,557,143, German Offenlegungsschriften (German Published Specification) Nos. 2,107,657, 2,453,936 and 2,453,967 and U.S.S.R. patent specification 400,597).
It is also known to prevent or cure iron deficiency diseases in plants by adding water-soluble iron salts, for example iron sulphate, to the substrate in which the plants grow. Using such conventional agents, it is indeed possible to supply the plants with iron in weakly acid substrates or in substrates having a neutral reaction. However, their use in soils having a weakly basic reaction suffers from considerable disadvantages. Thus, in weakly alkaline substrates, the iron ions cannot be taken up by the plants at all or can be taken up only in an insufficient amount because these ions then separate out in the form of sparingly soluble hydroxides and therefore do not contribute to plant nutrition.
Furthermore, it is known that, if needed, the plants can be supplied with iron in the form or iron chelate complexes of citric acid, gluconic acid, nitrilotriacetic acid, ethylenediaminetetracetic acid and ethylenediamine-N,N'-di-(o-hydroxyphenyl)-acetic acid. See "Der Vegetationsversuch" ("Vegetation Experiments") in "Methodenbuch" ("Book of Methods"), Volume VIII, Neumann Verlag, Radebeul, Berlin, 1951, 180 to 194; Plant Physiology 26, 411 (1951); Soil Science 80, 101 to 108 (1955) and "Organic Sequestering Agents", John Wiley and Sons, Inc., New York, 1959, 455 to 469.
With the aid of such iron complexes it is possible to supply the plants with iron not only in weakly acid or neutral soils but also, to a certain degree, in weakly alkaline soils because, as a result of the relatively high stability of these complexes, an undesired precipitation of the iron cations in the neutral or weakly basic medium is largely prevented. Nevertheless, the use of iron chelate complexes for the indicated purpose suffers from some disadvantages. Thus, the duration of action of iron chelate complexes of citric acid or gluconic acid is only relatively short, since these naturally occurring acids can be degraded relatively rapidly by soil micro-organisms. The iron chelate complexes of the synthetic aminopolycarboxylic acids with the exception of the iron complex of ethylenediamine-N,N'-di-(o-hydroxyphenyl)-acetic acid, which is important for combating chlorosis, can be employed only with certain limitations in strongly alkaline soils because the stability of the complexes does not always suffice to avoid the iron cations being immobilised in the form of sparingly soluble hydroxides or oxides. A further disadvantage is that the aminopolycarboxylic acids form very stable, highly toxic and at the same time water-soluble chelate complexes with the heavy metal ions of cadmium, lead and mercury, which can be present in the soil in the form of almost insoluble compounds. Since these heavy metal ion complexes can, because of their good solubility, pass into the soil water, the use of aminopolycarboxylic acids is not without risk for toxicological reasons. It is true that the iron complex of ethylenediamine-N,N'-di-(o-hydroxyphenyl)-acetic acid is, as already mentioned, of practical importance in combating chlorosis; however, it is a disadvantage that this compound can be prepared only with relative difficulty and is furthermore not light-stable.